"If you hadn't trumped my ace, I'm sure declarer would have guessed the hearts, as you have already shown a shortage in clubs, therefore you are more likely to have the longer trumps."
West accepted this accolade with due modesty, but confided afterwards, "I only did it so that I could play an early spade through that inviting- looking SAQ in dummy."
North opened One Club (Precision, 16+ points), and South responded One Heart (at least 5 hearts and 8+ points). They did well not to go overboard, and the final contract was Four Hearts.
West led the ten of clubs, which was overtaken by East with the jack, dummy playing low. East continued with the king, then the ace.
West elected to ruff his partner's ace then led a spade. As you can see, declarer had no concerns about the spade suit, but needed to locate the queen of trumps.
The play up until that point gave declarer the impression that if anyone held the guarded heart queen, it was more likely to be East, because surely West would not use up a trump on a trick his partner had already won, from an initial holding of Qxx. Winning the spade lead in hand, declarer played a small heart to the king, getting the bad news when East showed out.
South might well have misguessed the trumps if left to his own devices, but West's play pointed him firmly in the wrong direction.
Game all; dealer North
!K 10 8 2
#K Q 5 4
2Q 8 3
410 6 5 4 49 8 7 3 2
!Q 6 4 3 !Void
#J 8 3 #10 9 6 2
210 9 2A K J 7
!A J 9 7 5
26 5 4 2